Scientist Stephen Hawking fears aliens in giant spaceships may one day terrorise earth.

The physicist said extraterrestrials almost certainly exist on other planets.

Professor Hawking, 68, an expert on the universe who wrote A Brief History of Time, warns that jealous aliens could one day want to steal Earth's resources.

And, in a scenario similar to Hollywood blockbuster movie Independence Day, he fears contact with them might be catastrophic.

He said: "We only have to look at ourselves to see how intelligent life might develop into something we wouldn't want to meet.

"I imagine they might exist in massive ships, having used up all the resources from their planet. Such advanced aliens would perhaps become nomads, looking to conquer and colonise whatever planets they can reach.

"If aliens ever visit us, I think the outcome would be much as when Christopher Columbus first landed in America, which didn't turn out very well for the Native Americans." He makes the warnings in a new TV series highlighting his thoughts on big mysteries of the universe.

Prof Hawking says aliens probably will not just exist on planets or be roaming through space but will also live in the centre of stars.

He said with 100 billion galaxies, each with hundreds of millions of stars, Earth is highly unlikely to be the only planet with life: "The numbers alone make thinking about aliens perfectly rational," he said. "The real challenge is to work out what aliens might be like."

Prof Hawking believes most of them will be the equivalent of microbes or simple animals.

In his Discovery Channel documentary, two-legged herbivores are dramatically picked off by flying, yellow lizard-like predators.

And fluorescent aquatic animals form vast shoals in the oceans of a moon orbiting Jupiter.

Prof Hawking, who is almost completely paralysed by a form of muscular dystrophy and who communicates through a voice synthesiser, concludes that contact with aliens is "a little too risky".